Many investigations have suggested an immunoregulatory role for cyclic nucleotides, but the precise nature of this role has yet to be defined. Investigation of cyclic adenosine 3',5' monophosphate (cAMP) levels in control and mitogen stimulated lymphocytes have yielded conflicting results. Many of these inconsistencies may be due to a failure to control for and monitor lymphocyte subpopulations such as T and B lymphocytes. There is also evidence suggesting that the absolute level of cAMP may not accurately reflect the degree of cAMP-dependent protein kinase (cAPK) activation. Since cAMP is believed to elicit its physiological responses through cAPK activation it would appear that direct measurements of these enzymes would be a more sensitive indicator of cAMP function. The present studies are designed to avoid these two major objections to past studies by carefully separating and monitoring T and B lymphocytes and by measuring cAPK activity directly. The cAPK enzymes will be investigated in purified resting T and B lymphocyte preparations from the standpoint of their distribution between lymphocyte subpopulations, their physical-chemical properties, and their intracellular localization. In addition these same parameters will be investigated in mitogen stimulated lymphocytes. Intracellular localization will be achieved through the development of an immunocytochemical procedure for subcellular localization of cAPK. These studies will be important in testing the hypothesis that cAMP regulates lymphocyte function via translocation of activated cAPK into the nucleus where it may combine with chromatin macromolecules to regulate gene expression.